Enable SearchableSnapshotIT to run during build (opensearch-project#5937

) (opensearch-project#5939)

Previously this test required the feature flag to be set as a system
property, otherwise the test did nothing. This change programmatically
sets the feature flag to allow the test to run during all builds.
I ran the following command overnight (probably at least a couple
thousand iterations) on a dev machine and saw no failures, so I have
confidence this test is not flaky:

```
run-one-until-failure ./gradlew ':server:internalClusterTest' --tests "org.opensearch.snapshots.SearchableSnapshotIT"
```

Signed-off-by: Andrew Ross <andrross@amazon.com>

Signed-off-by: Andrew Ross <andrross@amazon.com>
(cherry picked from commit 901cd69)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Signed-off-by: Andrew Ross <andrross@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

server/src/internalClusterTest/java/org/opensearch/snapshots/SearchableSnapshotIT.java

@@ -6,7 +6,6 @@
 
 import com.carrotsearch.randomizedtesting.generators.RandomPicks;
 import org.hamcrest.MatcherAssert;
-import org.junit.BeforeClass;
 import org.opensearch.action.admin.cluster.node.stats.NodesStatsResponse;
 import org.opensearch.action.admin.cluster.snapshots.create.CreateSnapshotResponse;
 import org.opensearch.action.admin.cluster.snapshots.delete.DeleteSnapshotRequest;
@@ -43,19 +42,16 @@
 
 public final class SearchableSnapshotIT extends AbstractSnapshotIntegTestCase {
 
-    @BeforeClass
-    public static void assumeFeatureFlag() {
-        assumeTrue(
-            "Searchable snapshot feature flag is enabled",
-            Boolean.parseBoolean(System.getProperty(FeatureFlags.SEARCHABLE_SNAPSHOT))
-        );
-    }
-
     @Override
     protected boolean addMockInternalEngine() {
         return false;
     }
 
+    @Override
+    protected Settings nodeSettings(int nodeOrdinal) {
+        return Settings.builder().put(super.nodeSettings(nodeOrdinal)).put(FeatureFlags.SEARCHABLE_SNAPSHOT, "true").build();
+    }
+
     @Override
     protected Settings.Builder randomRepositorySettings() {
         final Settings.Builder settings = Settings.builder();